In this section, we focus on the consensus process in permissionless blockchain networks. In a permissionless blockchain network, every node has the opportunity to perform a transaction, propose a block of transactions or verify and validate the same. The communication in all permissionless blockchains' consensus protocols is the variant of the same in Byzantine Fault Tolerance (BFT) consensus process [1]}. However, for cryptocurrency-based permissionless blockchains, the functioning of the BFT protocol is modified. There are several consensus protocols such as Proof of Work (PoW), and Proof of Stake (PoS), which are examples of such variations. Here, we only discuss the major consensus protocols that are adopted especially by the cryptocurrency-based blockchain and provide an overview of different threats and illicit activities that have happened in such cryptocurrency-based blockchain. For the detailed study of the same, in [2]}, the authors survey different consensus protocols used in different blockchains. They highlight the performance and scalability issues in different protocols and emphasize the need to mitigate these issues for enabling widespread adoption. For the scope of our paper, we provide a brief overview.
